1. Field of the Invention
This invention relates to a heat fixer roll which is used in the contact heat fixing system of a dry type electric photocopying machine.
2. Description of the Prior Art
The most common process in the electric photocopying machine is the formation of the image over a photosensitive plate as an electrophotograph using a pigment-containing thermoplastic resin powder, known as a toner. The toner image is transferred to the copy paper and then fixed. As one method for fixing the toner image, a contact heat fixer system is often applied. In this contact heat fixer system, a heated fixer roll whose roll body surface has been covered with a layer of a heat-resistant releasing material and a pressure roll whose roll body surface has been covered with a heat-resistant elastomer are pressed together and brought into contact under a constant pressure to fix the fused toner on the paper with the passage of the copy paper between the two rolls.
The known heat-resistant releasing materials for heat fixer rolls are polytetrafluoroethylene and heat-resistant rubbers such as silicone rubbers, fluorine rubbers, and chlorosulfonated polyethylene rubbers such as Hypalon. However, the following undesirable phenomenon occurs. When the toner image is fused and fixed with the heat fixer roll, part of the toner image supported over the copy paper adheres on the surface of the heat fixer roll. As a result, when an incoming copy paper is brought into contact with the heat fixer roll, toner image partially transferred from a prior copy paper is partially transferred to the incoming copy paper and, at the same time, part of the toner image from this incoming copy paper adheres on the heat fixer roll. This phenomenon is generally called the offset phenomenon in printing technology. Such offset is an undesirable phenomenon in the process of fixing toner images. In order to prevent the occurrence of the offset phenomenon, the most commonly applied technique is the application of silicone oil over the heat fixer roll.
In such a contact heat fixer system, the surface of the heat fixer roll will be elastic since a coating of a heat-resistant rubber such as a silicone rubber as the heat-resistant releasing material has been applied over the surface at a constant thickness.
An advantage of a rubber coated roll is that unfixed toner is less frequently flattened resulting in excellent image quality. However, when a heat-resistant rubber is used and particularly in the case of silicone rubber, swelling of the rubber by the silicone oil added to prevent the offset is a significant problem. The heat resistance and abrasion resistance are significantly impaired by such swelling. For this reason, roll life is limited to 20,000 to 100,000 copies.
On the other hand, since a heat fixer roll which has been coated with polytetrafluoroethylene exhibits poor elasticity, problems occur in that the toner image is flattened during the fixing process so that image quality is stiff and in that neutral tints in photographic copying cannot be clearly fixed.
A common drawback of a polytetrafluoroethylene-coated heat fixer roll or a heat fixer roll coated with a heat-resistant rubber such as a silicone rubber is that the copy paper becomes entangled around the roll due to charging of the silicone oil added for the prevention of the offset phenomenon, due to charging between the heat fixer roll and the copy paper, or the pressure roll and the copy paper.
In an attempt to solve these drawbacks, separation pawls are generally attached with the heat fixer roll and pressure roll as a countermeasure. The inventors earnestly carried out the present investigation in an attempt to overcome the drawbacks of the conventional techniques and this invention was achieved as a result.